In the 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) system, User Equipment (UE) acquires Channel State Information (CSI) of a downlink channel according to a downlink reference signal, and then feeds the acquired CSI back to an eNodeB (eNB). According to the CSI fed back by the UE, the eNB determines the modulation and coding scheme and the physical resource location and the transmission mode of data sent to the UE. In a LTE system, there are three kinds of CSI: Channels Quality Indication (CQI), Pre-coding Matrix Indicator (PMI) and Rank Indicator (RI).
In the LTE system, UE feeds back the CSI of the downlink channel in two ways, which includes periodic feedback and aperiodic feedback, and the two ways are referred to as periodic CSI reporting and aperiodic CSI reporting respectively. The so-called periodic CSI reporting means that the UE feeds back CSI periodically according to the configuration by the eNB; the so-called aperiodic CSI reporting means that the eNB triggers the UE to feed back CSI for one time through a triggering signaling. For the periodic CSI reporting, generally speaking, if the UE has not sent a Physical Uplink Shared Channel (PUSCH) on a certain subframe, the CSI is transmitted on a Physical Uplink Control Channel (PUCCH); if the UE has sent the PUSCH, the CSI is born on the PUSCH for transmission. For the aperiodic CSI reporting, the CSI is only transmitted on the PUSCH.
In the LTE system, the eNB schedules a certain UE to send PUSCH through an uplink grant signaling. The uplink grant signaling can be born in a Downlink Control Information (DCI) format 0, and it is sent to the scheduled UE by a Physical Downlink Control Channel (PDCCH). 1 bit in DCI format 0 is used to trigger the UE to perform the aperiodic CSI reporting, which is called as CQI request.
If the UE detects the PDCCH which is with DCI format 0 and belongs to the UE, the UE sends the PUSCH according to the UL grant contained in the PDCCH. If the UE detects that the 1 bit CQI request is set as “1” in the DCI format 0, the aperiodic CSI reporting is sent in the PUSCH scheduled by the DCI format 0.
Besides the CQI request, the uplink scheduling information in the DCI format 0 further includes: 1 bit hopping flag used to indicate hopping enabled; resource block distribution and hopping resource distribution, 5 bits MCS Index (IMCS); 1 bit New data indicator (NDI); 2 bits TPC command for scheduled PUSCH; 3 bits Cyclic shift for DM RS and so on.
Wherein, 5 bits MCS Index (IMCS) is used to indicate jointly the Modulation and Coding Scheme (MCS) and Redundancy Version (RV), as shown in Table 1. The contents jointly indicated by IMCS include modulation order Q′m, Transport Block Size (TBS) index ITBS and redundancy version rvidx.
TABLE 1MCS index of PUSCH and corresponding modulation order, transportblock size index and redundancy versionRedundancyMCS IndexIMCSModulation OrderQm′TBS IndexITBSVersionrvidx020012102220323042405250626072708280929010210011410012411013412014413015414016415017416018417019418020419021619022620023621024622025623026624027625028626029Reserved1302313
The UE determines the modulation order Qm and Transport Block Size (TBS) according to the modulation order Q′m, the TBS index ITBS and related signaling and rules corresponding to the MCS index IMCS.
In the LTE system, when CQI request is 1 and IMCS is 29 in the DCI format 0, and the bandwidth of the scheduled PUSCH is: NPRB≤4, then aperiodic CSI reporting is triggered, and no transport block bearing an Uplink Shared Channel (UL-SCH) is transmitted in the scheduled PUSCH.
Additionally, a type of special UL grant, which is called as Random Access Response Grant (RAR grant) signaling, is born in the PDSCH to be sent to the corresponding UE which initiates the random access. In the non-contention based random access procedure, RAR grant also includes 1 bit CQI request used to trigger the aperiodic CSI feedback. If the UE, which initiates the random access, detects the RAR grant belonging to the UE, the PUSCH is sent based on this, and the PUSCH is called as First scheduled UL transmission, which is also called as Message 3 (Msg.3) during random access. If the UE detects that the 1 bit CQI request is set as “1” in the RAR grant, the CSI, which is fed back aperiodically, is sent in Msg.3. In the contention based random access procedure, the 1 bit CQI request is reserved in the RAR grant, and is not used to trigger the aperiodic CSI feedback.
According to feedback type of CQI and PMI, periodic/aperiodic CSI reporting respectively have multiple kinds of reporting modes, as shown in Table 2 and Table 3. Each kind of reporting mode corresponds to certain CSI reporting contents and reporting format. Which kind of reporting mode UE will adopt is indicated by parameters configured through higher-layer signalling (parameters cqi-FormatIndicatorPeriodic and cqi-ReportModeAperiodic indicates respectively periodic/aperiodic CSI reporting mode), and it is related to downlink transmission modes configured for the UE, i.e. under a particular transmission mode, only the corresponding CSI reporting mode can be configured.
TABLE 2Periodic CSI reporting mode and corresponding CQI/PMI feedback typePMI Feedback TypeNo PMISingle PMIPUCCH CQIwideband CQIMode 1-0Mode 1-1Feedback TypeUE Selected subband CQIMode 2-0Mode 2-1
TABLE 3Aperiodic CSI reporting mode and corresponding CQI/PMI feedback typePMI feedback typeNo PMISingle PMIMultiple PMIPUSCH CQIWideband CQIMode 1-2FeedbackUE selectedMode 2-0Mode 2-2Typesubband CQIHigher Layer-Mode 3-0Mode 3-1configuredsubband CQI
LTE-Advanced (LTE-A) system is the next generation evolved system of LTE system, and it extends transmission bandwidth by carrier aggregation (CA) technology to support bigger peak transmission rate. FIG. 1 illustrates the carrier aggregation of LTE-A system according to related technology. As shown in FIG. 1, each aggregated carrier is called as “component carrier (CC)”. Multiple CCs can be located in the same frequency band and can also be located in different frequency bands.
During carrier aggregation, there is one primary component carrier (PCC), which is also called as primary cell (Pcell) of the UE, in the multiple CCs configured or activated for the UE by the eNB, and other CCs configured for the UE are called as secondary component carriers (SCCs) or secondary cells (Scells) of the UE. Channel quality of the PCC is usually thought to be better. In the related art, it is proposed that UE can only send PUCCH and semi-persistently scheduled (SPS) PUSCH on its PCC, but it can not send the PUCCH or SPS PUSCH on other SCCs.
In the LTE-A system, an eNB can send signals to the same UE on multiple (downlink) CCs, therefore, the UE needs to feed back CSI of each CC which is configured or activated when the eNB configures or activates multiple (downlink) CCs for the UE. In the related art, it is proposed that each (downlink) CC can be configured individually with downlink transmission mode and CSI reporting mode for the same one UE. Then, the UE feeds back the CSI corresponding to CC according to the CSI reporting mode of each (downlink) CC. In the related art, two CSI reporting methods are also proposed under the circumstance of carrier aggregation.
Method I, UE feeds back the CSI reporting of one (downlink) CC at a time;
Method II, UE feeds back the CSI reporting of multiple (downlink) CCs at a time.
Feedback delay can be reduced if CSI reporting of multiple (downlink) CCs is fed back at a time, thereby improving downlink performance, and overheads of triggering signalling can be also saved for aperiodic CSI feedback. However, when CSI of multiple (downlink) CCs is fed back simultaneously, the feedback overhead will be increased, and under certain circumstances, it may cause redundant CSI reporting and may also influence the reliability of CSI reporting and uplink data transmission. Therefore, under the circumstance of carrier aggregation, it is an urgent problem how the UE performs channel state information reporting on multiple (downlink) CCs, which can not only ensure the reliability of channel state information transmission but also reduce the feedback delay of channel state information as soon as possible.
In addition, it is also proposed in the related art that cross-carrier scheduling is supported in the LTE-A system, i.e. scheduling information of other CCs is sent on one CC, and the scheduling information is born in the PDCCH with particular DCI format to be sent to the scheduled UE, and carrier indicator field (CIF) is introduced into the DCI Format to identify the scheduled CC. Currently, 5 CCs can be aggregated in LTE-A system at most, and CIF is 3 bits. Whether the cross-carrier scheduling is performed and whether the CIF is included in the DCI format are configured by high-layer signalling.